Cellulose degrading bacteria are present in considerable number in ceca of rats fed low- and high-fiber diets; however, in the ceca of rats fed a high-fiber diet the numbers of cellulolytics is 10-100 times greater. The organism isolated from ceca of rats fed high-fiber diets is a Ruminococcus species that requires for growth one or more nutrients found in rat intestinal fluid; the requirement(s) is a small molecule. In addition to cellulolytics, other organisms have been isolated and it has been found that the flora of the small intestine, and especially the cecum, of animals fed high-fiber diets is more complex than that of animals fed low-fiber diets. Further isolation, as well as nutritional and physiological characterization of both cellulolytics and non-cellulolytic organisms from rats fed both diets is proposed; in order to determine whether the presence of "dietary fiber" ca result in a different microbial flora that is capable of fiber degradation and that may play a direct role in the beneficial effects of fiber in the diet.